The present invention relates to an imaging system using coherent light radiation to expose a layered member in an image configuration and, more particularly, to a method for modifying an imaging member to suppress optical interference occurring within said photosensitive member which results in a defect that resembles the grain in a sheet of plywood in output prints derived from said exposed photosensitive member when the exposure is a uniform, intermediate-density gray.
There are numerous applications in the electrophotographic art wherein a coherent beam of radiation, typically from a helium-neon or diode laser is modulated by an input image data signal. The modulated beam is directed (scanned) across the surface of a photosensitive medium. The medium can be, for example, a photoreceptor drum or belt in a xerographic printer, a photosensor CCD array, or a photosensitive film. Certain classes of photosensitive medium which can be characterized as "layered photoreceptors" have at least a partially transparent photosensitive layer overlying a conductive ground plane. A problem inherent in using these layered photoreceptors, depending upon the physical characteristics, is the creation of two dominant reflections of the incident coherent light on the surface of the photoreceptor; e.g., a first reflection from the top surface and a second reflection from the bottom surface of the relatively opaque conductive ground plane. This condition is shown in FIG. 1; coherent beams 1 and 2 are incident on a layered photoreceptor 6 comprising a charge transport layer 7, charge generator layer 8, and a ground plane 9. The two dominant reflections are: from the top surface of layer 7, and from the top surface of ground plane 9. Depending on the optical path difference as determined by the thickness and index of refraction of layer 7, beams 1 and 2 can interfere constructively or destructively when they combine to form beam 3. When the additional optical path traveled by beam 1 (dashed rays) is an integer multiple of the wavelength of the light, constructive interference occurs, more light is reflected from the top of charge transport layer 7 and, hence, less light is absorbed by charge generator layer 8. Conversely, a path difference producing destructive interference means less light is lost out of the layer and more absorption occurs within the charge generator layer 8. The difference in absorption in the charge generator layer 8, typically due to layer thickness variations within the charge transport layer 7, is equivalent to a spatial variation in exposure on the surface. This spatial exposure variation present in the image formed on the photoreceptor becomes manifest in the output copy derived from the exposed photoreceptor. FIG. 2 shows the areas of spatial exposure variation (at 25.times.) within a photoreceptor of the type shown in FIG. 1 when illuminated by a He-Ne laser with an output wavelength of 633 nm. The pattern of light and dark interference fringes look like the grains on a sheet of plywood. Hence the term "plywood effect" is generically applied to this problem.
One method of compensating for the plywood effect known to the prior art is to increase the thickness of and, hence, the absorption of the light by the charge generator layer. For most systems, this leads to unacceptable tradeoffs; for example, for a layered organic photoreceptor, an increase in dark decay characteristics and electrical cyclic instability may occur. Another method, disclosed in U.S. Pat. No. 4,618,552 is to use a photoconductive imaging member in which the ground plane, or an opaque conductive layer formed above or below the ground plane, is formed with a rough surface morphology to diffusely reflect the light.
According to the present invention, the interference effect is eliminated by breaking up the coherence of reflections from the surface of the ground plane by a novel process which, in a preferred embodiment, includes forming the photoreceptor substrate (ground plane) by an electroforming process which imparts to the ground plane a matte-like finish. More particularly the present invention related to a process for forming a photosensitive imaging member comprising the steps of forming a ground plane with a matte finish by an electroforming process, and overlying said ground plane with at least a charge transport layer and charge generating layer.
Disclosures which are believed to be relevant to the present invention:
Application Ser. No. 07/546,214, filed on June 24, 1990, discloses a method for merging scanned beams from 2 or more diodes at a photoreceptor surface. The beams are at different wavelengths producing an exposure variation pattern at the surface which compensates for the plywood exposure.
Application Ser. No. 07/541,655, filed on June 21, 1990, discloses an imaging member with a ground plane formed on an underlying substrate whose surface has been roughened. The ground plane surface has a conforming roughness and presents a diffused reflecting surface to eliminate direct reflection causing the plywood exposure.
Application Ser. No. 07/546,990 discloses various processes for forming a ground plane with a rough surface morphology.
Application Ser. No. 07/552,200 discloses an imaging member having a low reflection layer formed on the ground plane. The low reflection layer reduces the secondary reflections from the ground plane contributing to the plywood effect.
Application Ser. No. 07/523,639, filed on May 15, 1990, discloses an imaging member which has a ground plane formed of a low reflection material. The ground plane serves to suppress the interference fringes caused by the otherwise strong reflections from a high reflecting ground plane.